/Engine/Source/Runtime/Engine/Classes/Materials/MaterialExpressionStrata.h
class UMaterialExpressionStrataHorizontalMixing : public [UMaterialExpressionStrataBSDF](API\Runtime\Engine\Materials\UMaterialExpressionStrataBSDF)
FExpressionInpu...
Background
Substrate material
uint32:  1
bUseParameterBlending
Merge Background and Foreground into a single material by mixing their inputs rather than their evaluation.
FExpressionInpu...
Foreground
Substrate material
FExpressionInpu...
Mix
Lerp factor between Background (Mix == 0) and Foreground (Mix == 1).